In the past, thermal cyclers utilized resistive heaters to heat a liquid and a compressor to cool the liquid. To eliminate the need for external compressors or water sources, thermoelectric heat pumps were experimentally used to alternately cool and heat thermoconductive blocks. The use of thermoelectric heat pumps at first proved to be unreliable under thermal cycle extremes. Recent developments in thermoelectric heat pumps have extended the reliability of the thermoelectric cooler devices, but their relatively short mean time before thermal stress induced failure increased their cost of use. Thus, thermal cyclers using water cooled units became available for those who require long term device reliability for applications which do not require very low temperatures, and thermoelectric heat pump thermal cyclers became available for those who prefer the stand alone feature of Peltier units (heat pumps) and for those who work at temperatures below 15.degree. C. Peltier junction heat pumps provide rapid heat transfer to and from a sample holding block with ramping rates up to 1.degree. C. per second within a 0.degree.-100.degree. C. temperature range. Programmable controllers have been added to the thermoelectric heat pump thermal cyclers cooling of RNA and DNA.
The problem with known thermal cycler devices using thermoelectric coolers is their low mean time to stress failure which increases their life cycle cost effectiveness.
Typically, thermoelectric heat pumps, or coolers, or modules, as they are often referred to in common parlance cannot withstand the high thermally induced stresses when the cooler is repeatedly cycled from heating to cooling as required, for example, in applications for DNA replication and duplication. In such applications, the sample vials are in contact with one side of the thermoelectric cooler which is cooled by applying a positive voltage. After reaching the desired cold temperature at a preselected ramp rate, the polarity of the voltage is reversed and the sample viles are heated to temperatures as high as 100.degree. C. at the preselected ramp rate. The process, or cycle, is then repeated until a sufficient size sample of target modules sufficient size is produced for DNA testing. As a result of the thermally induced stresses, thermoelectric elements (legs) of the cooler soon break loose from the soldered conductor pads and create high resistance joints within the cooler. In some cases, the thermoelectric elements move off their conductive pads to short with adjacent thermoelements. This failure is the principal contributor to the low mean time between failure of these devices.